In general, an electric double layer device is a device that stores electrical energy, such as a battery, a capacitor, or an electrolytic condenser. An electric double layer device is electrically charged and discharged using electrodes that are electrically conductive. Electric double layer devices are used in cellular phones, GPS receivers, MP3 players, and backup memory devices. In addition, electric double layer devices are used in wind energy systems, solar energy systems, and motors of electric vehicles and hybrid electric vehicles.
One example of such an electric double layer device may be a capacitor.
In the electric double layer capacitor, an electrostatic layer is formed at the interface between an activated carbon electrode and an organic electrolyte, and the state of an electric double layer functions as a dielectric in order to accumulate electricity in the same manner as a battery.
In particular, charges that accumulate in an electric double layer, formed between a solid electrode and a solid-state or liquid-state electrolyte, are used.
A capacitor has lower energy density than a battery. However, the capacitor is superior to the battery in terms of power density, that is, instantaneous high output. In addition, the capacitor is usable hundreds of thousands of times. That is, the lifespan of the capacitor is semi-permanent. For these reasons, capacitors are used in various fields.
The electric double layer capacitor is operated according to the following principle. When direct current voltage is applied to a pair of solid electrodes in the state in which the solid electrodes are placed in an electrolyte ion solution, negative ions are electrostatically drawn to an electrode acting as a positive electrode, and positive ions are electrostatically drawn to an electrode acting as a negative electrode. As a result, an electric double layer is formed at the interface between each electrode and the electrolyte. In particular, activated carbon has a plurality of pores. For this reason, the electric double layer is easily formed.
The electric double layer capacitor includes electrodes, a separator, an electrolyte, current collectors, and a case.
The selection of materials for the electrodes is most important when configuring the capacitor. However, the capacitance of the capacitor is also changed by various other components of the capacitor.
The materials for the electrodes must have high electrical conductivity and a large specific surface area. In addition, the materials for the electrodes must be electrochemically stable.
Another example of such an electric double layer device may be a battery.
The battery is a device that converts chemical energy, stored in a chemical material (i.e. an active material) contained therein, into electrical energy through an electrochemical oxidation-reduction reaction (a redox reaction).
The battery is constituted by assembling two or more electrochemical cells. Alternatively, the battery may be constituted by a single cell. The battery is configured such that electrons flow to the outside along a conductive wire due to an electrochemical reaction, rather than a chemical reaction. The electrons that flow along the conductive wire become a source of electrical energy, thereby being electrically useful.
More specifically, the battery has a positive electrode (cathode) active material and a negative electrode (anode) active material coated on respective current collectors. The positive electrode and the negative electrode are separated from each other by a separator. In addition, the positive electrode and the negative electrode are placed in an electrolyte, which enables the transfer of ions between the two electrodes.
In order to operate an electric lamp, an apparatus, an instrument, etc., the electrode materials and the electrolyte must be selected appropriately and arranged so as to have a specific structure such that sufficient voltage and current are generated between the two electrodes of the battery.
For example, a positive electrode, the positive electrode active material of which is reduced by electrons received from an external conductive wire, a negative electrode, the negative electrode active material of which is oxidized so as to emit electrons to the conductive wire, an electrolyte, which enables material to move such that the reduction reaction of the positive electrode and the oxidation reaction of the negative electrode are chemically harmonious, and a separator, which prevents physical contact between the positive electrode and the negative electrode, must be arranged so as to convert chemical energy into electrical energy based on interactions therebetween.
The negative electrode of the battery, arranged as described above, basically emits electrons while being oxidized, and the positive electrode receives electrons while being reduced (together with positive ions). When the battery is operated in the state of being connected to an external load, therefore, the two electrodes are electrochemically changed to thus perform electrical work.
At this time, the electrons, which are generated by the oxidation reaction of the negative electrode, move to the positive electrode via the external load, and then undergo a reduction reaction with the positive electrode active material. Consequently, the flow of charges is completed as the result of movement of negative ions (anions) and positive ions (cations) toward the negative electrode and the positive electrode in the electrolyte.
In this way, the reaction is performed in the electrolyte such that charges continuously flow in the external conductive wire, and the electrical operation is performed using the charges.
Based on the kind of electrolyte, the battery may be classified as a liquid electrolyte battery or a polymer electrolyte battery. In general, the liquid electrolyte battery is referred to as a lithium ion battery, and the polymer electrolyte battery is referred to as a lithium polymer battery.
FIG. 1 is a schematic view showing the structure of a general electric double layer device, FIG. 2 is a schematic view illustrating the principle whereby an electric double layer capacitor manufactured using a general electric double layer device is charged, and FIG. 3 is a circuit diagram illustrating the principle whereby the electric double layer capacitor manufactured using the general electric double layer device is charged and discharged.
As shown in FIG. 1, the general electric double layer device, denoted by reference numeral 100, includes electrodes 10, an electrolyte 20, current collectors 30, a separator 40, a first lead terminal 61, and a second lead terminal 62.
On the assumption that the electric double layer device 100 is a battery, the chemical energy of a chemical material (i.e. an active material) contained therein is converted into electrical energy through an electrochemical oxidation-reduction reaction, and the electrodes 10 have a positive electrode active material and a negative electrode active material, which are coated on the respective current collectors 30.
Describing the characteristics of the electric double layer device 100 in more detail based on the assumption that the electric double layer device 100 is a capacitor, on the other hand, energy is stored using the distribution of positive and negative charges which are arranged within a short distance from each other at the interface between the two different electrodes 10, the capacitance, in farads, is high, and the change and deterioration in performance upon repeated charge and discharge cycles thereof are very low.
The electrodes 10 are made of activated carbon, which has a large specific surface area, and store charges generated at the electric double layer, which is disposed at the interface with the electrolyte 20. Of the electrical characteristics of the electrodes 10, capacitance and internal resistance are the most important criteria for evaluating the performance thereof. Consequently, the electrodes 10 must exhibit low specific resistance and have a porous structure. In the porous structure, the size and distribution of pores must be simple and biased within a predetermined range. The material characteristics of the electrodes 10 most strongly affect the inherent charge and discharge characteristics of the electric double layer capacitor.
In recent years, therefore, an activated carbon-based material, which has a large specific surface area and is inexpensive, has been mainly used as the material for the electrodes 10, and research into the use of metal oxides and conductive polymers in order to increase energy density has been increasingly conducted.
Meanwhile, an organic solvent, quaternary ammonium salt (organic), or a sulfuric acid solution (aqueous) is used as the electrolyte 20. For the organic solvent electrolyte, polycarbonate (PC) and ethyl methyl carbonate (EMC) or PC and dimethoxyethane (DME) may be mixed at a predetermined ratio in order to improve electrical conductivity.
An electric double layer capacitor 100 using an organic electrolyte has a capacitance per unit area of 4 to 6 μF/cm2. The electrical conductivity of the organic electrolyte is higher than that of the aqueous electrolyte. Consequently, an electric double layer capacitor 100 using an aqueous electrolyte has a capacitance per unit area of 5 to 10 μF/cm2, which is higher than that of the electric double layer capacitor 100 using the organic electrolyte. However, the electric double layer capacitor 100 using the aqueous electrolyte has problems in that the potential window is narrow and decomposition occurs.
Nonwoven fabric, porous polyethylene (PE), or porous polypropylene (PP) film is used as the separator 40.
The principle whereby the electric double layer capacitor is charged is as follows. As shown in FIG. 1, the two electrodes 10, which are placed in the electrolyte 20, are disposed so as to be opposite each other in the state in which the separator 40 is located therebetween. In the state in which electrical energy is not supplied from the outside, as shown in FIG. 2, which is a schematic view illustrating the principle whereby the electric double layer capacitor is charged, the electric double layer capacitor is in a bulk state, in which charges are non-uniformly distributed. As a result, the potential difference between the electrodes 10 becomes 0. When electrical energy is supplied from the outside, as shown in FIG. 3, which is a circuit diagram illustrating the principle whereby the electric double layer capacitor is charged and discharged, charges are uniformly distributed throughout the electric double layer capacitor. As a result, as shown in FIG. 2, energy having a voltage corresponding to a potential difference of 2Φ1 is charged between the two electrodes 10.
At this time, even when the supply of electrical energy is interrupted, the electric double layer, which has already been formed, is not extinguished, and therefore the charged electrical energy is retained.
Related Art Document (Korean Patent Application Publication No. 2009-0118328 Entitled Module Type Electric Double Layer Capacitor and Method of Manufacturing the Same)
FIG. 4 is a view showing a process of manufacturing an electric double layer capacitor according to the Related Art Document, FIG. 5 is a view illustrating a method of manufacturing an integrated electric double layer capacitor according to the Related Art Document, and FIG. 6 is a view illustrating a process of manufacturing an electrode device that constitutes the electric double layer capacitor according to the Related Art Document.
In general, a secondary battery that can be charged and discharged, for example, an energy storage apparatus, such as an electrolytic condenser or an electrochemical double layer capacitor (EDLC), is configured to have a wound type structure, e.g. a jelly-roll type structure.
As shown in FIG. 4, a wound type energy storage apparatus, such as a wound type electrochemical double layer capacitor, generally includes a cylindrical case 20 made of aluminum (Al) and a wound device 10 mounted in the case 20.
The wound device 10 is formed by winding a strip-shaped electrode stack, that is, positive and negative electrode devices with an electrolyte interposed between the positive and negative electrode devices, into a cylindrical shape and then taping the outside of the strip-shaped electrode stack in order to prevent the strip-shaped electrode stack from being unwound.
The wound device 10 formed as described above is impregnated with an electrolyte and is then mounted in the cylindrical case 20. A terminal plate 30 is disposed above the wound device 10, and lug type or screw type external terminals 40 are fastened to the terminal plate 30.
In addition, a neck 21, which prevents the terminal plate 30 from being pushed downward, is formed in the upper part of the case 20 in a recessed state. The wound device 10 is mounted in the case 20 after the neck 21 is formed in the case 20. The wound device 10 is electrically connected to the external terminals 40 via terminals 120. Subsequently, the upper end 22 of the case 20 is curled. As a result, the terminal plate 30 is fixed in the case 20, and the assembly process is completed.
Referring to the upper figure of FIG. 6, an electrode device 100 includes an electrode current collection sheet 111 made of general aluminum foil and an electrode active material 112 coated on the electrode current collection sheet 111.
The electrode active material 112 is formed by applying conductive paste including mostly activated carbon.
A terminal 120 is coupled to the electrode device 100. Specifically, a portion of the electrode device 100 to which the terminal 120 will be coupled is scratched to remove the electrode active material 112 therefrom, the scratched portion of the electrode device 100 is drilled, and the terminal 120 is coupled to the drilled portion of the electrode device 100 by riveting.
The applicant of the present application has improved the electric double layer device according to the Related Art Document, and therefore proposes an electric double layer device according to the present invention.